1. Field of the Invention
The present invention relates generally to image-forming devices, and more particularly, to the cooling of a toner cartridge in an image-forming device.
2. Description of the Related Art
Image forming devices such as laser printers utilize a light beam that is focused to expose a discrete portion of a photoreceptive or image transfer drum in order to attract printing toner to these discrete portions. One component of a laser printer is the photoreceptive drum assembly. The photoreceptive drum assembly is made out of photoconductive material that is discharged by light photons, typically emitted by a laser. The drum is initially given a charge by a charge roller. As the photoreceptive drum revolves, the printer directs a laser beam across the surface to discharge certain points. In this way, the laser “draws” the letters and images to be printed as a pattern of electrical charges—an electrostatic latent image. The system can also work with either a more positively charged electrostatic latent image on a more negatively charged background, or on a more negatively charged electrostatic latent image on a more positively charged background.
The printer's laser or laser scanning assembly draws the image to be printed on the photoreceptive drum. A known laser scanning assembly may include a laser, a movable mirror, and a lens. The laser receives the image data defined by pixels that make up the text and images one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every pixel to be printed. Typically, the laser does not actually move the beam. Instead, the laser reflects the light beam off a movable mirror. As the mirror moves, the light beam passes through a series of lenses. This system compensates for the image distortion caused by the varying distance between the mirror and points along the drum. The laser assembly moves in one plane horizontally as the photoreceptor drum continuously rotates, so the laser assembly can draw the next line. A print controller synchronizes this activity. In the process of forming the latent image on the photoreceptive drum, the laser discharges those areas where the latent image is formed.
When the toner becomes electrostatically charged, the toner is attracted to exposed portions of the image transfer drum. After the data image pattern is set, charged toner is supplied to the photoconductive drum. Because of the charge differential, the toner is attracted to and clings to the discharged areas of the drum, but not to the similarly charged “background” portions. Toner is an electrostatically charged powder with two main ingredients, pigment, and plastic. The pigment provides the coloring, such as black in a monochrome printer, or cyan, magenta, yellow, and black in a color printer, and forms the text and images. The pigment is blended with plastic particles so the toner will melt when passing through the heat of a fuser assembly. The toner is stored in a toner cartridge housing, a small container built into a removable casing. The printer gathers the toner from a sump within the housing and supplies it to a developer unit using paddles and transfer rollers. The developer roll is a charged rotating roller, typically with a conductive metal shaft and a polymeric conductive coating, which receives toner from a toner adder roll positioned adjacent the developer roll. Due to electrical charge and mechanical scrubbing, the developer roll collects toner particles from the toner adder roll. A doctor blade assembly engages the developer roll to provide a consistent coating of toner along the length and surface of developer roll by scraping or “doctoring” excess toner from the developer roll. The doctor blade may also induce a charge on the toner. This, in turn, provides a consistent supply of toner to the photoconductive drum. When the coating of toner on the developer roll is inconsistent, too thick, too thin, or bare, the coating of the photoconductive drum is inconsistent, and the level of darkness of the printed image may vary due to these inconsistencies. This condition is considered a print defect.
The electrostatic image on the photoconductive drum is charged such that the toner particles move from the developer roll onto the latent image on the photoconductive drum to create a toned image on the photoconductive drum. The toned image is transferred from the photoconductive drum to a printable medium such as paper or onto a intermediate transfer belt which then transfers the toned image onto the printable medium. The paper or transfer belt is oppositely charged to the toner, causing it to transfer to the paper or transfer belt. This charge is stronger than the charge of the electrostatic image, so the paper or belt pulls the toner particles away from the surface of the photoconductive drum. Since it is moving at the same speed as the drum, the paper or transfer belt picks up the image pattern exactly.
One problem that often occurs in a laser printer or other image-forming device is toner leakage. Toner from the sump can leak into the toner cartridge and interfere with the proper operation of the unit. One significant area of toner leakage is a path along portions of the developer roll where a J-seal, positioned proximate both ends of the developer roll, slidably engages the developer roll, particularly where the developer roll, doctor blade, and J-seal all meet. These locations are difficult to seal due to the tolerances, stiffness, and deflections of the aforementioned components. Observations of operational toner pressure as well as vibration and drop testing have demonstrated that the areas around the surface of the developer roll and the J-seal are a frequent toner leak path, especially in higher volume housings.
The interface between the developer roll and the J-seal, identified on the developer roll as the “clean band,” creates heat inside the toner cartridge when the developer roll turns. Friction is unavoidable with current designs because the J-seal must contact the developer roll around its periphery at all times. The J-seal interface is a source of high friction because the J-seal must be made from a pliable material in order to securely contain the toner in the cartridge. The J-seal interface contacts the developer roll, which is frequently covered by a polymeric or rubberized material with a high coefficient of friction. It will be appreciated that the temperature of the developer roll along its length is significantly higher at the clean bands than it is at intermediate positions due to friction with the J-seal.
One solution to excessive heat from the J-seal interface has been to apply a lubricant to the clean band area in an attempt to decrease the coefficient of friction. However, such an approach has significant drawbacks. Any lubricant applied to the J-seal or to the ends of the developer roll can potentially contaminate the toner and ruin any printed image. Additionally, the lubricant can seep into other areas of the cartridge or printer, causing unwanted damage and interfere with the proper operation of the unit.
Another solution to excessive heat from the J-seal has been to utilize directed airflow, such as from a fan, to blow air across the entire length of the developer roll. However, this had been found to be ineffective in lowering the temperature of the developer roll by any significant amount.
In addition, the heat created by the friction at the J-seal interface causes further problems with the proper operation of a laser printer or other image-forming device as print speed increases. Since it is essential to maintain pressure between the J-seal and the developer roll, more heat is created as the print speed increases. In known printers, a print speed of 35 pages per minute (ppm) is slow enough that, even with continuous printing, the heat created at the J-seal can be dissipated into the surrounding cartridge parts and into the atmospheric air to prevent heat related failure. In such an instance, the toner cartridge can reach a thermal equilibrium and still operate properly with undirected machine airflow as a cooling method. However, printing at higher speeds such as at or above 50 ppm causes extreme overheating, which is localized at the ends of the developer roll around the J-seal interface. Low thermal conductivity of the developer roll worsens the heating condition, and a large temperature gradient can be created around the clean bands in the axial direction of the developer roll.
It will be appreciated that high temperatures negatively affect the ability of the J-seal to seal toner inside the cartridge. As heat from the clean band areas increases, the temperature of the surface of the developer roll increases, and the temperature of the toner in the immediate region also increases. Temperatures of up to 70° C. around the J-seal interface have been measured when a printer was operated at 50 ppm. For some toners, fusing can occur at approximately 46° C. Thus, it will be appreciated that toner fusing may occur in the area of contact between the J-seal and the developer roll when the image forming device is operated at speeds of 50 ppm or higher. In such an instance, the J-seal contacts an irregular layer of fused toner on the developer roll, and not an extremely smooth surface, which is the most desirable condition in order to achieve a consistent and reliable seal. This condition allows toner to escape past the J-seal and out of the toner cartridge.
Once toner leakage at the J-seal begins, toner loss almost always continues at a rapid rate, permitting several grams of toner per minute to escape into the printer. Such large amounts of toner losses are substantial enough to severely affect cartridge yield, and may result in yields of several thousand pages fewer than expected. In addition, major print defects occur, as the escaped toner from the toner cartridge can spill directly onto the transfer belt near the location of the first transfer or onto the print media.